Solid-state power-amplifier modules (SSPAs) have a variety of uses. For example, SSPAs may be used in satellites to amplify severely attenuated ground transmissions to a level suitable for processing in the satellite. SSPAs may also be used to perform the necessary amplification for signals transmitted to other satellites in a crosslink application, or to the earth for reception by ground based receivers. SSPAs are also suitable for ground-based RF applications requiring high output power.
Typical SSPAs achieve signal output levels of more than 10 watts. Because a single amplifier chip cannot achieve this level of power without incurring excessive size and power consumption, modem SSPA designs typically use a radial splitting and combining architecture in which the signal is divided into a number of individual parts. Each individual part is then amplified by a respective amplifier. The outputs of the amplifiers are then combined into a single output that achieves the desired overall signal amplification.
Additionally, a typical power-combiner, such as the in-phase Wilkinson combiner or the 90-degree branch-line hybrid, in which a number of binary combiners are cascaded, becomes very lossy and cumbersome when the number of combined amplifiers becomes large. For example, to combine eight amplifiers using a conventional, binary microstrip branch-line hybrid at Ka-band (˜26.5 GHz), the combiner microstrip trace tends to be about six inches long and its loss tends to exceed 3 dB. It should be understood that a 3-dB insertion loss means that half of the RF power output is lost. Such losses are unacceptable for most applications.
To overcome these loss and size problems, many approaches, including the stripline radial combiner, oversized coaxial waveguide combiner, and quasi-optical combiner, have been investigated. The stripline radial combiner, using multi-section impedance transformers and isolation resistors, still suffers excessive loss at Ka-band, mainly because of the extremely thin substrate (<10 mil) required at Ka-band. The coaxial waveguide approach uses oversized coaxial cable, which introduces moding problems and, consequently, is useful only at low frequencies. The quasi-optical combiner uses hard waveguide feed horns at both the input and output to split and combine the power. The field distribution of a regular feed horn is not uniform, however, with more energy concentrated near the beam center. To make field distribution uniform, these waveguide feed horns require sophisticated dielectric loading and, consequently, become very large and cumbersome.
It would be desirable, therefore, if there were available low-loss, low-cost, radial power divider/combiners that could be used in designing high-frequency (e.g., Ka-band) SSPAs.